1. Field of the Invention
The present invention relates to an image forming apparatus, and, more particularly, to an image forming apparatus in which at least two latent images formed on an image carrier are developed by two different types of developing agents which are charged to opposite polarities and the developed images are transferred onto a transfer medium so as to form a visible image. More specifically, the present invention concerns an image forming apparatus which is capable of forming an image in two colors using two developing agents.
2. Description of the Prior Art
Conventionally, printers which are used as terminals in information devices such as computers, facsimiles, or CAD's have been of either the electrophotographic type or electrostatic recording type.
In such printers, a laser beam, a light emitting diode, or a liquid crystal display is employed to produce on a charged photosensitive medium an electrostatic latent image which represents an information signal, or a recording electrode is used to produce on a dielectric medium an electrostatic latent image representing an information signal. The formed electrostatic latent image is made visible by a developing device, and the developed image is transferred onto a sheet of transfer paper. The transferred image is then fixed onto the transfer paper for achieving an observable recorded image. Such image recording is generally done in a single color, e.g., in black.
If the image is recorded in at least two colors, e.g., if calculated values or data values are recorded in a different color from that used for the format, or a portion of a drawing which is output by a CAD is in a color different from that of the remaining portion, the image will become clearer, so understanding of the contents thereof will be facilitated. In other words, recording an image at least in two colors is effective to provide quick and better comprehension of information. Accordingly, various types of electrophotographic or electrostatic recording image forming apparatus which are capable of forming an image in two colors have recently been proposed.
Among the known image forming apparatus of the above-described type, a method of forming an image using, as developing agents in two colors, two developers charged to opposite polarities is advantageous for prevention of mixture of the two colors. One example is disclosed in the specification of Japanese Patent Laid-Open No. 137538/1980. In the disclosed apparatus, a photosensitive medium which has been uniformly charged to a positive polarity is subjected to a first radiation, which represents a first information signal, so as to form a negative latent image, and the formed latent image is then reverse developed by a first positively-charged developing agent. Subsequently, the photosensitive medium is subjected to a second radiation, which represents a second information signal, so as to form a positive latent image, and the formed latent image is then normally developed by a second negatively-charged developing agent, thereby forming on the photosensitive medium first and second developed images which are charged to opposite polarities. Other examples are disclosed in the specifications of Japanese Patent Laid-Open No. 37148/1973 and Japanese Patent Laid-Open No. 81855/1979, where latent images at three different electric potential levels, i.e., an area at a predetermined reference electric potential, a first latent image at an electric potential which is lower than that of the reference potential, and a second latent image at an electric potential which is higher than that of the reference potential, are sequentially or simultaneously formed on a negatively-charged photosensitive medium. Subsequently, the first latent image is reverse developed by a first negatively-charged developing agent, and a second latent image is normally developed by a second positively-charged developing agent so as to form on the photosensitive medium first and second developed images which are charged to opposite polarities.
In either case, since the thus-formed first and second developed images have opposite polarities, they are charged by a charger prior to the transfer in such a manner that they have the same polarity, and are then transferred onto a sheet of transfer paper so as to form an image in two colors thereon.
When first and second developing agents which are charged to opposite polarities are used to form an image, the images developed by the first and second developing agents are charged in such a manner that they have the same polarity by a pre-transfer charger in the above-described manner. However, if the developing agent which is to be charged to an opposite polarity is a magnetic toner, the following disadvantage occurs: when the magnetic toner is charged by a pre-transfer charger, a magnetic substance, such as magnetite, dispersed in the magnetic toner particles leaks the charge applied by a high electric field, generating a difference in charge density between the edge portions Ta of a toner image T which are relatively easily charged and a central portion Tb which cannot be readily charged to the opposite polarity owing to the leakage of the charges, as shown in FIG. 3. Therefore, the electric field E applied during transfer is caused to concentrate at the edge portions Ta of the toner image T, and transfer failure thereby occurs at the central portion Tb of the toner image T, producing a transferred image which is hollowed at the center. This tendency to produce a non-uniform transferred image increases when the toner image consists of a line.
In the case of an image consisting of a line, image blurring also occurs due to scattering of the toner particles, prohibiting a vivid image from being produced.